WEATHEEINa OF BIOEITE 





oxidizing and solvent action of organic acids. The removal of 

 the magnesia, which must have existed mainly in the mineral 

 olivine, indicates that the decomposition has gone on even to 

 the production of carbonate of magnesia and the separation of 

 free silica and iron oxides. 



Weathering of Diorite. — An analysis by the present writer 

 of a closely related rock, a diorite, and its residual soil, from 

 North Garden, Albemarle County, Virginia, yielded the results 

 given in columns I and II below. The rock here was fine- 

 grained, of an almost coal-black color finely speckled with 

 whitish flecks due to the presence of feldspars. The microscope 

 showed it to be composed mainly of hornblende with interstitial 

 soda-lime feldspars and scattering areas of titanic iron. The 

 clay, or soil, to which it gave rise was deep brownish red in 

 color and highly plastic, though distinctly gritty from the pres- 

 ence of undecomposed minerals. In columns III, IV, and V are 

 given the loss and gain of the various constituents calculated 

 on an alumina constant basis. 



Analyses of Fresh and Becomposed Diorite from Albemarle Coxtntt, 



Virginia 







I 



11 



III 



IV 



y 



CONSTITXTBHTS 



Frksh 



DlOKITB 



Decom- 

 posed 



DiORITB 



Oaloitlatbd 



Loss FOE En- 



TiBE Book 



Pbe Cent 

 OF Each 

 Constitu- 

 ent SAVED 



Pee Cent 

 OF Each 



CONSTITTT- 

 ENT LOST 



Silica (SiOg) 



Alumina (AlgOs) . . . 

 Iron sesquioxide (¥6203) -^ 

 Lime (CaO) ..... 

 Magnesia (MgO) . . . 

 PotasK (K2O) .... 



Soda (NasO) 



Phosphoric acid (p20)5 . 

 Ignition 



46.75 % 



17.61 



16.79 



9.46 



5.12 



0.55 ' 



2.66 



0.25 



0.92 



42.44% 



25.51 



19.20 



0.37 



0.21 



0.49 



0.56 



0.29 

 10.92 



17.43% loss 

 0.00 « 

 3.53 *' 

 9,20 " 

 4.97 " 

 0.21 " 

 2.17 *' 

 0.00 

 0.00 



62.69 % 



100.00 



78.97 



2.70 



2.83 



61.25 



15.13 



80.11 



100.00 



37.31 % 



0.00 

 21,03 

 97.30 

 97.17 

 38.75 

 84.87 

 19.87 



0.00 





100.01 % 



99.99 % 



37.61% loss 



• ■ « « 



• « • « 



Weathering" of Andesites. — Results largely confirmatory of 

 these, but showing at the same time some interesting variations, 

 were obtained by J. B. Harrison from a study of the rocks and 



^All iron calculated as FcsOb. 



